Wireless communication systems support wireless communication among a number of terminals (often referred to as user equipment). Typically, each wireless terminal communicates with a base station over a wireless channel comprised of a forward link and a reverse link. The forward link (also known as the downlink) refers to communications from the base station to the user terminal, while the reverse link (also known as the uplink) refers to communications from the user terminal to the base station.
Orthogonal frequency-division multiplexing (OFDM) is a frequency-division multiplexing (FDM) scheme often employed in wireless communication systems. Closely-spaced orthogonal sub-carriers are used to carry data that is typically divided into several parallel data streams or channels (one for each sub-carrier). Each sub-carrier is modulated with a conventional modulation scheme at a low symbol rate, thereby maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth. LTE (Long Term Evolution) techniques have been proposed for a fourth generation (4G) of wireless communication systems with improved throughput and performance. In an LTE wireless network, wireless resources are assigned using frequency domain scheduling to the uplink and the downlink.
Channel estimation is an important issue in a wireless communication system, such as OFDM systems. Among other benefits, the obtained channel information allows coherent detection to be employed. In order to satisfy the throughput and performance demands of emerging wireless networks, sophisticated signal processing techniques are employed that require knowledge of the channel. For example, the independent sub-channels in OFDM systems improve transmitter performance by adjusting the bit rate and transmit power across sub-channels according to their channel conditions.
In an exemplary LTE system, the channel is estimated by processing reference information in the received OFDM symbols. For an exemplary LTE uplink, for example, each resource block is composed of 14 OFDM symbols. Symbols 4 and 11 carry reference signals that are used for channel estimation. The channel can be directly estimated at these two locations. In order to estimate the channel at other locations, however, the known channel components are interpolated or extrapolated to all other OFDM symbol locations to obtain the desired equalizer filter coefficients at other symbol locations. This process requires an interpolation, and consequently a matrix inversion, at every interpolated OFDM location, resulting in a rather high computational complexity.
A need therefore exists for improved techniques for interpolating channel components at known OFDM symbol locations to other OFDM symbol locations. A further need exists for methods and apparatus for improved wireless channel estimation using interpolation elimination in the Eigen domain.